Process and apparatus for making negative electrodes, in particular in cadmium or zinc, for electrochemical generators, and the negative electrodes thus obtained

ABSTRACT

A relatively little adherent and fragile powdery layer is deposited on a continuous metallic tape or strip by passing continuously said tape negatively polarized through an electrolytic bath contained in a vat and containing ions of the active metal to be deposited; in the bath is immersed an electrode made out of active metal and a compensation electrode; the powdery layer is consolidated by means of a calender. 
     Said process is applicable to the manufacture of cadmium or zinc negative electrodes for electrochemical generators.

This is a division of application Ser. No. 908,783 filed May 23, 1978.

BACKGROUND OF THE INVENTION.

The present invention relates to negative electrodes for electrochemicalgenerators, more particularly to negative electrodes made out of cadmiumor zinc for generators in an alkaline, neutral or acid medium.

As regards the cadmium electrodes used substantially as negative platesin alkaline nickel-cadmium and silver-cadmium generators, it is knownhow to prepare them by means of three types of processes (the third ofwhich is also applicable to the zinc electrodes), namely,

1. The process whereby a conductive porous support or core is repeatedlyimmersed, consisting in immersing a support, generally in sinterednickel, in melted cadmium nitrate, and thereafter in an aqueous solutionof an alkaline metal hydroxide which transforms said nitrate in cadmiumhydroxide. The pores of the cadmium hydroxide conductive support formingthe active material are thereby filled up. Of course, the two immersionoperations are repeated several times for providing a sufficient depositof active material so that the electrode reaches its required capacity,that is, in operation, is capable of storing the required electricityquantity.

Excellent cadmium electrodes are thereby obtained, but their cost ofproduction is very high.

2. The process whereby a non-porous conductive support or core is coatedwith a powder and consisting in coating, for instance, throughcompacting or impasting with a cadmium base and/or a cadmium compoundpowder, a non porous conductive support made, for instance, of ametallic gauze or a perforated plate.

This second process is more economical than the first, but theelectrodes which it provides cannot support high loading or unloadingconditions and their thickness cannot be very thin, which is often adisadvantage.

3. The process whereby cadmium and/or cadmium hydroxide is directlyelectro-deposited on a support or core, porous or non porous, consistingin cathodically polarizing said support while immersing it in a solutionof at least one cadmium salt.

Such a process provides at a relatively low cost, good quality cadmiumelectrodes withstanding high ratings, such as the electrodes obtainedthrough the first process, and which may be very thin.

Generally, according to how the electrodeposition is being performed,three types of electrolytic metallic deposits may be made:

(A) AN ADHERENT DEPOSIT, COMPACT AND WITHOUT POROSITIES, OBTAINABLETHROUGH CONVENTIONAL ELECTROPLATING; IT SHOULD BE NOTED THAT IT IS ALSOPOSSIBLE TO OBTAIN A ZINC DEPOSIT OF THIS TYPE; SUCH CADMIUM OR ZINCDEPOSITS ARE CONVENIENT FOR PROVIDING PROTECTIVE PLATINGS, BUT NOTNEGATIVE ELECTRODES, SINCE THEY RESUME THEIR PASSIVITY AS THE METAL ISNOT IN A DIVIDED STATE;

(B) AN ELECTROLYTIC DEPOSIT, ADHERENT AND POROUS, USABLE FOR MAKINGELECTRODES; A PROCESS OF THIS TYPE, WHICH IS THE OBJECT OF French Pat.No. 1,281,247 filed on July 6, 1960 by YARDEY INTERNATIONAL CORPORATON,consists in electrolytically depositing an active metal on the cathodicsurface from a highly ionized solution containing a large proportion ofions others than those of said active metal, the active metal ions beingpresent in solution in a proportion lower than about 10% by weight; theactive metal can be cadmium or zinc for forming negative electrodes, orsilver for forming positive electrodes; the porous deposits onelectrodes, in particular on negative electrodes, obtained by a processof such type offer the disadvantage of having a far lower reactive powerthan deposits in powder form obtained by other processes and, moreover,this reactive power decreases after repeated cyclings; therefore, theydo not provide very good quality electrodes;

(c) a powdery deposit, of fine texture and little adherent, usable forpreparing metallic powders; such powders are very reactive since verydivided, but the deposit offers the disadvantage of being littleadherent, heterogeneous and fragile, whereby it cannot be used forforming the active part of an electrode.

SUMMARY OF THE INVENTION

The present invention has for an object to provide such powdery depositswith a very fine texture, but adherent, homogeneous and not fragile,while being very reactive.

The invention has also for an object to provide a manufacturing processof a material coated with a deposit of an active metal for makingnegative electrodes for electrochemical generators, characterized inthat a powdery layer, relatively little adherent and fragile, isdeposited on a continuous metallic-tape or strip by passing continuouslysaid metallic tape, negatively polarized, through an electrolytic bathcontaining ions of the active metal to be deposited, the bath beingdisposed in an electrolysis vat with an electrode in said active metal,and preferably also an electrode in a non active metal, while ensuring,according to an important characteristic of the invention, a laminarflow of the bath in the whole area of the vat comprising said tape andsaid electrode or electrodes which are positively polarized, and saidpowdery layer being consolidated by subjecting said tape coated with thepowdery layer, when coming out from said bath, to a calenderingoperation.

It is a further object of the invention to provide a device forrealizing the deposit of a layer of an active metal on a continuous tapeor strip by using the aforementioned process and characterized in thatit comprises, on the one hand, an electrolysis vat comprising a mainelectrodeposition compartment containing an electrolytic bath with ionsof the active metal to be deposited into which dips an electrode of saidactive metal, and preferably also an electrode of non active metal, aninput compartment for the arrival of the electrolytic bath,communication means between said input compartment and saidelectrodeposition compartment for causing to flow passing in laminarfashion the electrolytic bath from the input compartment to theelectrodeposition compartment, an output compartment, means forproviding the overflow of the electrodeposition compartment into theoutput compartment, preferably means for recycling the electrolytic bathfrom the output compartment into the input compartment, means forcirculating continuously in the electrodeposition compartment andthrough the electrolytic bath contained therein the continuous tape uponwhich the active metal layer has to be deposited, and means fornegatively polarizing said tape and positively polarizing said electrodeor electrodes, and, on the other hand, a calender arranged in thevicinity of the electrodeposition department of the vat and means forforwarding said tape at the output of the electrodeposition compartmentthrough the cylinders of said calender.

Another object of the invention is to provide, as a new article ofmanufacture, a material coated with an active metal deposit, inparticular of cadmium or zinc, and adapted to constitute negativeelectrodes for electrochemical generators, characterized in that it hasbeen obtained by using the process hereabove defined, in particular inthe device also hereabove defined; this material is capable, inthicknesses lower than 1 mm, of possessing faradic capacitances reachingup 100 mAh/cm².

In any case, the invention will become more apparent from the followingdescription made in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE shows, in perspective, with parts torn away, a deviceaccording to the invention, given by way of a non limitative example,for practicing the process according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in the sole FIGURE, the device comprises firstly anelectrolysis vat 1 comprising four compartments, viz. anelectrodeposition compartment 2, an input and filling compartment 3, anoutput or overflow compartment 4 and a compartment 5 of a smallerdimension and called access compartment. The electrodepositioncompartment 2 communicates with the input compartment 3 through a numberof holes 6 formed in the bottom 7 of the electrodeposition compartment2; it communicates with the output compartment 4 through an overflowopening 8, formed in the upper portion of wall 9 separating compartments2 and 4; finally it communicates with the access compartment 5 via anelongated rectangular slot 10 extending across the bottom 7.

The unit formed by compartments 2, 3 and 5 is filled with an electrolyte(not shown) up to the level of hole 8 formed in wall 9; as regardscompartment 4, it is not filled up to that level since a recyclingsystem has been foreseen between the base of compartment 4 and the topof compartment 3, said recycling system comprising a piping 11communicating upstream with the lower portion of compartment 4, arecycling pump 12, a piping 13, a cooling chamber 14 and a piping 15emerging at 16 in the upper portion of compartment 3, the recyclingsystem having as effect not only to bring back the electrolyte from thebottom of the output compartment 4 up to the upper portion of the inputcompartment 3, but also to cool the electrolyte which has a tendency ofheating up when the electrolysis vat 1 is operating. The circulation ofthe electrolyte is shown by a simple arrow.

The electrolyte is an acid or an alkaline electrolyte containing ions ofan active metal; thus, it may be formed of a dilute aqueous solution ofcadmium sulfate and sulphuric acid when cadmium is to be deposited or adilute solution of zinc oxide in a concentrated aqueous solution ofpotash for forming a zinc deposit.

The material to be coated is formed of a continuous tape or strip 17 fedby a supply spool 18 on which it is wound. The tape 17 passes acrosscompartment 5 filled with the electrolyte but not subjected to the linesof electric field, then it penetrates through slot 10 in theelectrodeposition department 2 in which it receives the active metaldeposit.

On the other hand, tape 17 is negatively polarized by sources 23a and23b of the continuous current due to a guide 19 connected by conductors20, 20a, 20b to the negative poles of sources 23a and 23b.

In compartment 2 are arranged two electrodes, namely the main electrode21a in active metal, that is in cadmium or zinc, and an auxiliaryregulation electrode 21b in an inert metal compatible with the medium,for instance in lead in an acid medium (sulphuric acid and cadmiumsulfate) for the deposit of cadmium, in nickel in alkaline medium(potash and zinc oxide) for the zinc deposits. The electrodes 21a and21b are positively polarized by being respectively connected throughconductors 22a and 22b to the positive poles of sources 23a and 23brespectively. The function of electrode 21a in active metal is tomaintain the concentration of active metal ions of the electrolyte (notshown) which is in the vat, by playing the role of a soluble anode,while the electrode 21b acts as a regulation means.

During the electrolysis, the dissolution efficiency of the soluble anode21a is in fact of the order of 100%, whereas the efficiency of thedeposit on the tape is only of a %, for instance of 80% in the case ofcadmium and 95% in the case of the zinc deposit. Under such conditions,if the same current intensity was applicable to the soluble anode 21aand to the tape 17, there would be an accumulation of active metal inthe bath. Due to the existence of two sources 23a and 23b and of theelectrode 21b in a metal which is not partaking to the electrodepositionprocess, said disadvantage is being mitigated; to this effect, andsupposing a efficiency of 80% for the deposit of active metal, 20% ofthe intensity of the current applied to tape 17 is applied to electrode21b, whereas the 80% of said current intensity will be applied to thesoluble electrode 21a; more generally, if the electrodepositionefficiency was of a %, the source 23a would apply a % of the currentintensity applied to tape 17, to the soluble electrode 21a, while thesource 23b would apply (100-a) % of the current intensity applied totape 17, to the inert electrode 21b, the total current intensity appliedto tape 17 corresponding to 100%.

On the other hand, hydrogen is liberated from tape 17 in compartment 2,said liberation having the advantage of making powdery the active metaldeposit on tape 17; the electrode 21b, in inert metal, oxidizes the bathwater, whereby an oxygen discharge appears on its surface with thegeneration of hydrogen ions, the latter being produced in a quantityequal to that of the hydrogen ions which are eliminated in the form ofhydrogen gas on the surface of tape 17.

Finally, the device of the single FIGURE comprises also a calender 24comprising two rotary cylinders 25 and 26 arranged just abovecompartment 2 in vat 1, the tape 17a coated with an active metal depositbeing driven by the rotation of cylinders 25 and 26. The gap betweencylinders 25 and 26 compresses further the active deposit deposited onthe tape thereby providing the final tape 17b which is usable for makingnegative electrodes for electrochemical generators. In the drawing hasbeen shown with double arrows the advance direction of the tape.

The operation of the device just described is the following.

The calender 24 (or any other convenient means) draws tape 17, 17a inthe direction of the double arrow in succession through guide 19 whichpolarizes it cathodically, the access compartment 5 in which it iswetted by the electrolyte, the compartment 2 in which it is subjected tothe electrolysis phenomenon and receives a spongy deposit of cadmium,and between the cylinders 25 and 26 of calender 24 in which the depositis being compressed and consolidated. Tape 17b with its consolidatedcadmium or zinc layer can be thereafter subjected to various operations,in particular to an improvement chemical treatment, to a washing and/ora drying, for instance as described in the French patent applicationfiled this same day by the Applicant for a "process for improving thequality of electrodeposited negative electrodes for electrochemicalgenerators and negative electrodes obtained by said process".

As regards more particularly the electrolysis, said electrolysisconsists, as already discussed:

in a dissolution of anode 21a with passage of active metal in theelectrolyte with an efficiency of nearly 100%;

an emission of oxygen at anode 21b;

the formation of hydrogen ions at anode 21b;

a deposit of active metal, cadmium or zinc, on tape 17 with anefficiency lower than 100%;

a hydrogen emission on the spongy deposit of active metal.

For the reasons hereabove explained, it is possible to regulate theactive metal content of the electrolyte. On the contrary, there is aloss of water through electrolysis with emission of hydrogen on tape 17awith its deposit, and an emission of oxygen on the inert electrode 21b.It is therefore necessary to provide the controlled addition of water,for instance, by means of an electrovalve (not shown) which introducesin vat 1, 0.3357 grams of water per amp-hour having flown across inertelectrode 21b.

Due to the inert electrode 21b and the electrovalve, there is providedan excellent regulation of the electrolyte composition.

As regards the tape, it may be made of a full or perforated metallicplate, in expanded metal, in metallic gauze, the metal being forinstance nickel, nickelled iron, silver, or zinc.

Two examples of how the invention can be practiced will now be given inorder to provide respectively cadmium and zinc deposits.

EXAMPLE 1 : active cadmium deposit

The tape or strip 17 is a pure nickel tape, perforated, of a thicknessof 0.1 mm and a width of 14 cm.

The electrolyte is constituted by a dilute aqueous solution of cadmiumsulfate and sulphuric acid with 10 to 100 g/l of Cd⁺⁺ (for instance 20g/l and from 10 to 100 g/l (for instance 50 g/l) of H₂ SO₄.

The current density is within 50 to 1000 mA/cm², for instance 230 mA/cm.

The temperature of the electrolyte is comprised between 5° C. and 75°C.; it is a function of the current density (for instance 18° C. at 230mA/cm²).

The tape speed is of the order of one meter per hour (for instance 2m/h) and that of the electrolyte bath of the order of one meter perminute (for instance 1.5 m/mn).

The distribution of the anodic current is: 80% on the cadmium solubleanode 21a and 20% on the lead inert anode 21b.

The thickness of the cadmium deposit at the output of vat 1 is of 3 mmabout; the calendering reduces said thickness to 0.5 mm, that is acompacting rate of over 80%, the apparent density of the depositedcadmium being lower in this example than about 34% of the massivecadmium density.

EXAMPLE 2: active zinc deposit

The tape or strip 17 is made of expanded zinc.

The electrolyte is constituted by a zinc oxide and potash aqueoussolution containing from 5 to 10% of ZnO and from 30 to 45% of KOH (forinstance 45% of KOH and 8% of ZnO).

The current density is between 50 and 1000 mA/cm² (for instance 200mA/cm²).

The electrolyte temperature lies between 5° C. and 75° C.; its dependson the current density (for instance 25° C. at 200 mA/cm²).

The distribution of the anodic current is of 95% on the zinc solubleanode 21a and 5% on the nickel inert anode 21b.

The thickness of the zinc deposit at the output of vat 1 is of 4 mm;calendering reduces said thickness to 0.2 mm. The compacting rate inthis example is over 90% and the apparent density of the deposit of zincis lower in this example than about 26% of the massive zinc density.

There is provided by the process of a invention the deposit of a porousactive layer having an excellent behaviour on its support, the assemblyof the deposited layer and support being cuttable for forming a negativeelectrode, eventually after an improvement treatment according to theprocess which is the object of the aforementioned patent application.

Obviously and as can be assumed from the previous description, theinvention is in no way limited to such application modes and embodimentswhich have been more specially considered; on the contrary, itencompasses all their modifications.

We claim:
 1. A device for providing the deposit of an active metal layeron a continuous tape, characterized in that it comprises on the onehand, an electrolysis vat comprising a main electrodepositioncompartment containing an electrolytic bath with ions of the activemetal to be deposited in which is immersed an electrode in said activemetal, an input compartment for the arrival of the electrolytic bath,communication means between said input compartment and saidelectrodeposition compartment for causing the electrolytic bath to flowin a laminar fashion from the input compartment to the electrodepositioncompartment, an output compartment, means for providing the overflow ofthe electrodeposition compartment into the output compartment, means forcirculating continuously in the electrodeposition bath, through theelectrolytic bath contained therein, the continuous tape on which thelayer of active metal is to be deposited, and means for negativelypolarizing said tape and positively polarizing said electrode, and onthe other hand, a calender arranged in the vicinity of theelectrodeposition compartment of the vat and means for passing said tapeat the output of the electrodeposition compartment through the cylindersof said calender.
 2. The device according to claim 1, wherein it furthercomprises means for recycling the electrolytic bath from the outputcompartment to the input compartment.
 3. The device according to claim1, wherein the main compartment comprises further a non active metalelectrode immersed into the electrolytic bath, said deposit comprisingalso means for positively polarizing said non active metal electrode,the ratio between the anodic current supplied by the means forpositively polarizing the active metal electrode and the anodic currentsupplied by said means and the means for positively polarizing the nonactive metal electrode being substantially equal to theelectrodeposition efficiency of the active metal on the tape in the vat.4. The device according to claim 1, wherein the electrodeposition vatcomprises an electrovalve capable of introducing continuously water inthe electrolytic bath in a quantity substantially equal to 33 grams peramp-hour having flowed through said non active metal electrode.
 5. Thedevice according to claim 1, wherein the vat provides a vertical laminarflow, the three compartments of the vat are arranged side by side, themain compartment communicating with the input compartment through anumber of holes formed in the bottom of the main compartment and withthe output compartment through at least one overflow opening formed atthe upper portion of the wall separating the main compartment from theoutput compartment.
 6. The device according to claim 1, wherein the vatcomprises a fourth compartment communicating through a slot with themain compartment and through which is carried the tape, said means fornegatively polarizing the tape being arranged at the input of saidfourth compartment.
 7. The device according to claim 1, wherein theactive metal is cadmium.